The processes for separation by adsorption of a gas from a gaseous mixture containing it, by adsorption of this gas, are well known. These processes use a cyclic pressure variation and are generally designated as PSA processes (for "Pressure Swing Adsorption"). These processes are particularly used for separating nitrogen from air, for the recovery of other gases from air, principally oxygen. This type of cyclic processes, comprises two principal phases. A first phase consists in a selective adsorption of a gas of the gaseous mixture on an adsorbent mass and passage of the other gases through this mass. The adsorption phase is effected at a high pressure. The second phase consists in a desorption of the gas previously adsorbed. This phase is effected at a relatively lower pressure. When the low pressure is below atmospheric pressure, one generally then speaks of a VSA (for "Vacuum Swing Adsorption"). However, for facility, there will be designated hereafter as "PSA process" not only PSA processes properly so-called, but also VSA processes.
The phases of adsorption and desorption are repeated cyclically for the continuous treatment of a flow of gaseous mixture. Numerous variations of this process comprising additional phases, have been developed, and are well known to those skilled in the art. To this end, reference could be had for example to the patent applications EP-A-350.373; WO-A-91/12874; EP-A-108.005; FR-A-2.579.484; FR-A-2.612.083 and FR-A-2.624.759.
Conventionally, PSA processes are carried out by means of devices comprising different adsorbent masses arranged each in a plurality of vessels, with or without auxiliary capacity.
The adsorbents generally used consist in molecular sieves or zeolites of the type zeolite 13X, 5A and 10X or mordenites.
Other adsorbents have been described as being able to be used in a PSA process. Thus, U.S. Pat. No. 3,140,933 and European application EP-A-297.542 relate to the utilization of zeolites X, which is to say zeolites whose Si/Al ratio is comprised between 1 and 1.5, strongly exchanged with lithium. These zeolites permit improving the selectivity of adsorption of the nitrogen contained in a gaseous mixture, such as air. Zeolites exchanged with lithium have however the large drawback of being of high cost. This cost could be up to four times that of a conventional zeolite. Because of this, the use of these zeolites in a conventional PSA device increases considerably the capital cost.
In this regard, reference can for example be had to U.S. Pat. No. 5,268,023, which states in effect that the zeolites exchanged with lithium with an exchange rate higher than 88% have too high a cost of production. This cost is principally connected to the large quantities of lithium salts necessary for their preparation, these lithium salts themselves being very expensive. This patent thus proposes the use of zeolites X exchanged with lithium at rates comprised between 70 and 88%, these zeolites having an Si/Al ratio around 1. However, the preparation of such zeolites requires substantially the same quantity of lithium salts as that necessary to prepare zeolites according to for example European application EP-A-297.542. Thus, the lithium ions are associated with tetrahedric AlO.sub.2 units which are constituents of the zeolite. However, the smaller is the Si/Al ratio, the larger is the quantity of tetrahedric AlO.sub.2 units in the zeolite. Because of this, the quantity of salts required to achieve a given exchange rate is higher, the lower is the Si/Al ratio.
So as to make the cost of zeolites exchanged with lithium acceptable, it has also been proposed in European application EP-A-461.478 to use them in a particular PSA process, for the separation of nitrogen from air, and recovery of the oxygen. According to this process, the ratio between the high pressure and the low pressure of the cycle must be comprised between 1.4:1 and 4:1. Thus, it is possible to decrease the energy consumption required to effect the adsorption phases. The energy saving thus effected is considered to compensate the cost of the zeolites exchanged with lithium.
However, this process requires the use of a mass of adsorbent which can be up to two times greater than that required in a PSA process in which the ratios of the high and low pressures are outside the range contemplated by EP-A-461.478.
The adsorbent mass being constituted of zeolites exchanged with lithium, it will be understood that the overall capital cost of the PSA device for practicing this process is very high and that it can be compensated by the saving in energy only after a very long time of operation.
Moreover, the range of pressure ratios from 1.4:1 to 4:1 cannot be used directly in a conventional PSA device, in which the compression pumps particularly are adapted to pressure ratios located outside this range. It is thus necessary to modify the compression pumps of a conventional PSA device for practicing the process according to EP-A-461.478.
It appears that the practice of a PSA process with zeolites exchanged with lithium has not yet been able to be carried out without substantially raising the overall capital cost of the PSA device.
French patent applications 90.12538 and 92.04487 in the name of the applicant, which correspond to U.S. Pat. Nos. 5,133,784 and 5,441,559, describe new PSA processes and devices according to which the adsorbent mass is not arranged in vessels but in a rotating adsorber. Within the scope of the present invention, by "rotating adsorber" is meant a rotating device comprising at least one adsorbent material arranged in a plurality of separated masses about and secured to a rotatable support shaft about a vertical axis.
For the separation of gases from a mixture of gas by means of a rotating adsorber, the gaseous mixture is caused to circulate radially through the adsorbent mass which is rotating at a predetermined speed. Each time this mass effects a complete rotation, it is successively subjected to different phases of a PSA process as mentioned above.
Such rotating adsorbers as well as their use for the separation of gases from a gaseous mixture, are described in detail in the French patent applications cited above, whose disclosures are incorporated herein by reference in their entirety.
The adsorbents used to the present in this type of rotating adsorber are those used in conventional PSA processes. These adsorbents are therefore essentially zeolites 5A, 13X and 10X or mordenites.
The rotating adsorbers have the known advantage of permitting an increase in the productivity relative to a conventional PSA device, the productivity being expressed in Nm.sup.3 of gaseous product per hour and by m.sup.3 of adsorbent (Nm.sup.3 /h/m.sup.3); one Nm.sup.3 being one m.sup.3 of gas measured at 0.degree. C. and at atmospheric pressure. But they have the drawback, however, of leading to a diminution of recovery (expressed in volume of gas produced relative to the volume of gaseous mixture treated).